Raise translation limit for inclusion in release

Follow-up of !59 (merged)

We shouldn't include every single translation language, since some of them have only a few percent of our strings translated.

I chose 50% as a limit for now, which I also used for GTK iirc. Later on we should probably increase this. The Localization Lab people recommend 100% and reviewed, but I find this too harsh, given that it's all run by volunteers. Briar Android is using 80%, iirc, which sounds like a good choice for a first stable release.